1. Field of the Invention
The invention is generally related to intake systems for seawater desalination systems and is specifically directed to a desalination intake system having a net positive impact on habitat.
2. Discussion of the Art
Fish and larvae entrapment and entrainment losses are a key environmental issue for desalination plants. Desalination plants are often located in ecologically sensitive coastal estuaries. The juvenile fish larvae, which are abundant in these waters, are killed when they are entrained or entrapped in desalination plant intake systems.
Screened intake systems have been developed for power plants that reduce entrainment and entrapment, but these cannot always be successfully applied at industrial waterfront sites. These sites are optimal locations for large scale desalination plants due to the large demand for high quality water. In addition, even the best screen system with fish return capability is only able to reduce entrainment by 85-90% versus an unscreened open ocean intake. This still results in a significant loss of fish and larvae due to the high concentration of sea life in the near shore environment.
Travelling screens with fine mesh (0.5 mm) have been used in once through seawater cooled power plants. These travelling screens achieve about an 85% removal efficiency. However, these systems require a fish return system that routes the recovered fish and larvae away from the intake system. For once through power plant cooling water, the fish and larvae can be routed to the discharge cooling water or a separate fish channel. These are typically located a significant distance away from the intake to prevent re-ingestion of the discharge cooling water or fish.
Once through power plants use large flows and low temperature rises (about 10° F.). Thus, the returned fish and larvae can survive in the discharge cooling water, or in a fish discharge channel, which is near the cooling water discharge.
Desalination plants have a discharge stream that has a high brine concentration. In addition it may contain anti-scalant and water treating chemicals. Any returned fish or larvae must be discharged away from the inlet and away from the discharge line. This makes placement of the intake, discharge and fish return especially difficult in an industrial area where seafront acreage is limited. Intake and outfall pipelines have been used; but, these are expensive and may interfere with navigation (dredged ship channels).
Travelling screens also have a high mortality rate for fish and larvae impinged on the screen and subsequently returned. Overall mortality rates of about 50% are typical for Gulf of Mexico water temperatures, even for modified travelling screens with fish buckets. The stress of impingement and reduced oxygen content in the water cause this high mortality.
Angled screens with sweeping water flow to a bypass fish channel have been effective in reducing mortality in river applications. The sweeping flow and bypass channel allow the fish and larvae to pass by the face of the screen without becoming impinged. However, typical seawater sites have alternating weak tidal currents, which are insufficient to sweep the fish by the face of the screen.
Wedgewire passive screens have been proven to be about 85-90% effective in removing fish and larvae from seawater intakes. However, in order to achieve this effectiveness, the following conditions must be met:
1) Small opening size (about 0.5 mm)
2) Slow velocity through the opening (about 0.5 ft/s)
3) Significant sweep velocity across the face of the screen (>1 ft/s)
The first two conditions require significant screen surface area. For large desalination plants, this can be impractical due to site restrictions. This is especially true for industrial or ship channel locations where waterfront real estate is limited.
The third condition also is difficult to achieve in seawater conditions since tidal currents are alternating. Depending on location, the tidal currents may not reliably generate the sweeping velocities needed to prevent entrainment and entrapment on the screen.
Subsurface intakes use horizontal or vertical beach wells to supply seawater to the desalination plant. Subsurface wells are effective at preventing entrainment and entrapment since the sea floor acts as an effective filter, thereby removing essentially all sea life. However, subsurface intakes require a high porosity sea bed to provide a sufficient flow of seawater to support a commercial desalination plant. At many locations the sea bed porosity is too low to support a commercial desalination unit. In addition, there is a long term risk of damaging coastal aquifers with salt water intrusion.
Many of the world's estuaries are stressed due to reduced fresh water flows. On the U.S. Gulf Coast, this has led to oyster reef habitat destruction. In addition to producing oysters, oyster reefs provide habitat for juvenile fish. Oysters are attacked by parasites (dermo-protozoan, oyster drill—snail) when insufficient spring flood freshwater pulses enter the estuary. Upstream dams on the rivers feeding the estuaries are typically constructed to capture the spring floodwater for agricultural, municipal, and industrial use. Although minimum flows are supplied to the estuary on a year round basis, the cleansing effect of a spring flood event is no longer available.
Ship channels have also been dredged through estuarial bays. This facilitates commerce, but can increase estuary turbidity and channel tidal flows. Fertilizer runoff also enters the estuary in higher concentrations due to the reduced inlet water flows. The reduced tidal flows, higher fertilizer concentration, and higher turbidity can lead to hypoxic conditions in the estuary. This leads to additional oyster reef habitat destruction.
It remains, therefore, desirable to provide a seawater intake system that can be employed in commercial desalination systems near shorelines where the fresh water is required with a minimum of environmental impact on the fragile sea life dependent upon the coastal waters.